Many disorders, e.g., brain disorders and certain types of paralysis, as well as reduction of restenosis during angioplasty procedures, are treated by an electrical stimulus or local drug delivered to specific sites in the brain or other parts of the body. One shortcoming of conventional treatment devices and procedures is that conventional treatment devices are large and their placement may cause damage to the patient or that the release of drug at a desired pace is impossible or difficult. Another shortcoming of conventional treatment devices and procedures is that the devices are often implanted and must remain connected to the outside world for their control signal or energy supply. In addition, in conventional devices and procedures, drug delivery to the target tissue may cause trauma to the patient and may not be precisely delivered to the target tissue, and, where there is a physical connection to the outside world, the repeated trauma to the patient required by repeated invasion and introduction of foreign objects into the tissue increases the likelihood of infection. Yet another shortcoming of conventional devices and procedures is that they do not provide precise timing of the delivery of the electrical stimulus or drug in response to phenomena happening in, and to stimuli generated by, the tissue or the organ being treated indicating the need for the delivery of such an electrical stimulus or drug.
In recent years, controlled drug delivery systems utilize an implant made of a polymer, either natural or synthetic, that is combined with a drug or other active agent in such a way that the active agent is released in a pre-determined manner. For example, the release of the active agent may be constant over a long period, or it may be cyclic over a sustained period of time. It may be triggered by the environment or other external stimulus. In any case, the purpose behind controlling the drug delivery is to achieve more effective therapies while eliminating the potential for under-dosing or over-dosing.
There are three primary mechanisms by which active agents can be released from such a delivery system: diffusion, degradation, and swelling followed by diffusion. Each of these mechanisms, however, has disadvantages when employed as a delivery mechanism. In the case of diffusion, the disadvantage is that as the release progresses, the release rate will decrease, because the active agent concentration in the polymer decreases, and the agent has a progressively longer distance to travel, thus, the diffusion time to relevant tissue will increase. In the case of degradation, the process generates degradation by-products that may not be tolerated within the biological environment causing inflammatory reactions that may lead to adverse effects. Finally, for the swelling/degradation approach, drug release will be accomplished only when the polymer swells. The drug may prematurely be released upon contact of bodily fluid outside the target area. In addition, the drug may not be released at all if the target area does not contain a suitable environment to facilitate drug release, e.g., will not cause sufficient swelling. All adverse effect risks of the degrading polymer exist here also.
While advantages of these current drug delivery systems can be significant, their potential disadvantages or limitations cannot be ignored: the decreased rate of drug release over time, the possible toxicity or non-bio-compatibility of the materials used, undesirable by-products of degradation, and the potential for both under-dosing and overdosing. Thus, there remains a need for an improved drug delivery system that is inert, biocompatible, safe from accidental release, and capable of providing a consistent rate of drug release.